1. Technical Field
The present invention relates to an ink jet printer in which a plurality of nozzles jet minute ink drops of a plurality of colors and in which particles of ink (ink dots) are formed on a print medium to draw pre-determined characters and images.
2. Related Art
An ink jet printer generally provides low-cost and high-quality color printed material. As such, ink jet printers are widely used not only in offices but also by personal users due to the popularization of personal computers and digital cameras.
Generally, in ink jet printers, a moving part called a carriage, comprising ink cartridges and print heads, moves back and forth over a print medium in a direction perpendicular to the direction of the movement of the print medium while nozzles of the print head eject ink drops to form ink dots on the print medium. In this manner, pre-determined characters or images are drawn on the print medium to create the desired printed material. The carriage comprises ink cartridges for four colors including black yellow, magenta and cyan and a print head for each of the colors, so that not only monochrome print but also full color print in any combination of the respective colors can be easily performed. Print in six colors, including the colors light cyan and light magenta, seven colors and eight colors are also implemented.
In the ink jet printer described above, printing by moving the ink jet heads on the carriage back and forth in a direction perpendicular to the direction of the movement of the print medium, the ink jet heads must be moved back and forth approximately ten times or more to print a whole page. Therefore, it has the drawback of taking a longer time for printing than a print apparatus using another scheme; for example, a laser printer or a copying machine using an electrographic technique.
On the other hand, in an ink jet printer comprising ink jet heads (which may or may not be integrated) of the same length as the width of the print medium but not within a carriage, the ink jet heads do not need to be moved back and forth over the print medium, which enables quicker one-pass printing similar to the speed of a laser printer. An ink jet printer in the former scheme is generally called a “multi-pass (serial) ink jet printer”, while an ink jet printer in the latter scheme is generally called a “line head ink jet printer”.
The above types of ink jet printers are required to provide higher gradation. Gradation is the density of each color included in a pixel represented by an ink dot. The size of an ink dot depends on the density of a color of each pixel is called gradient, while the number of gradients represented by an ink dot is called the number of gradations. High gradation means that the number of gradations is large. To change gradient, it is necessary to change a drive pulse to an actuator provided for an ink jet head. For example, if an actuator is a piezoelectric element, when a voltage value applied to the piezoelectric element is large, the magnitude of displacement (distortion) of the piezoelectric element (precisely, a vibrating plate) is also large. This is used to change the gradient of an ink dot.
According to JP-A-10-81013, a plurality of drive pulses having different voltage peak values are combined and coupled to generate a drive signal. The signal is output to piezoelectric elements of nozzles for the same color provided for an ink jet head. According to the drive signal, a drive pulse for the gradient of an ink dot to be formed is selected for each nozzle. The selected drive pulse is supplied to a piezoelectric element of an appropriate nozzle to jet an ink drop. In this manner, the requested gradient of an ink dot is achieved.
A method of generating a drive signal (or drive pulse) is illustrated in FIG. 2 of JP-A-2004-306434. According to this method, data is read out from a memory for storing drive signal data, a D/A converter converts the data into analog data, and a drive signal is supplied to an ink jet head through a current amplifier. A current amplifier circuit comprises transistors in push-pull connection, as shown in FIG. 3 of the document, in which a linear drive amplifies a drive signal. However, in a current amplifier with such configuration, a linear drive itself of a transistor is inefficient. Moreover, such an amplifier has the drawback of large circuit size since the transistor itself should be large for a countermeasure against heat, or the transistor needs a cooling plate radiator. Particularly, the large size of the cooling plate radiator is a major obstacle to the layout.
To resolve the drawback, JP-A-2005-35062 discloses an ink jet printer for generating a drive signal by controlling the reference voltage of a DC/DC converter. According to the document, an efficient DC/DC converter is used to dispense with a radiating unit for cooling. Additionally, a PWM signal is used so that a D/A converter can be realized using a simple low-pass filter. These can be realized in a small circuit.
However, a DC/DC converter is designed to generate a constant voltage. As such, the head drive apparatus of an ink jet printer using the DC/DC converter in JP-A-2005-35062 has a problem in that in a waveform, for example, the necessary rapid rise and fall of a drive signal cannot be obtained for an ink jet head to jet ink drops well. Of course, the head drive apparatus of an ink jet printer in which a pair of transistors in push-pull connection amplifies the current of an actuator drive signal in JP-A-2004-306434 has the problem that a cooling plate radiator is so large that it cannot be actually laid out; particularly in a line head ink jet printer having a large number of nozzles, and as a result, a large number of actuators.